GS is considered safe at doses of up to 10 g per day; the recommended daily dose of 400 mg is unlikely to produce any toxic effects. Mild side effects, such as diarrhea, stomach cramps, and headache, were reported in one study that used a dosage of 600 mg daily. Whether these effects are dose-related remains unclear.[1] In some rodent studies, the amount of GS required to be fatal to half the studied rodents (the LD50) has been found to be as high as 3,990 mg of GS per kg of bodyweight (mg/kg).[2]
However, the safety evaluation of GS is complicated by a number of factors. Different preparations and different parts of the plant itself have different chemical compositions. This makes it difficult to research, as the available supplements are not all the same. Furthermore, toxicology data is limited and not well reported in most of the available studies, and there are very few studies focusing on safety in humans.[3] Additionally, there are a few case reports of drug-induced liver injury and hepatotoxicity which warrant further investigation.[4][5]
Hypoglycemia is another potential risk of GS supplementation, since GS is used to lower blood sugars. People with diabetes who are supplementing with GS in addition to their medication should be closely monitoring their blood glucose levels to avoid hypoglycemic episodes.[3]
It is thought that GS also impacts cytochrome P450, a protein in the liver that is important for drug metabolism. If this is the case, the use of GS could change the absorption, efficacy, and side effects of many different medications, perhaps most notably those used for diabetes treatment.[3]